Griddled Steak Burgers with Jarlsberg + Onions

cube steak, minced 1 pound ground chuck 2 tablespoon Montreal steak seasoning, divided 1 pinch crushed red pepper flakes 1/4 cup Homemade Steak Sauce, plus more for serving 1 large sweet onion, sliced 4 slices Jarlsberg cheese 4 pretzel hamburger buns